Generally, four, six, eight, or more equally spaced round rods assembled in a circle are used as an ion guide in high efficiency capture, transmission, and/or storage of ions in variety of mass spectrometers. In recent years, the use of such multipole ion guides have been practiced widely, especially in mass spectrometers(MS) interfaced with atmospheric pressure ionization (API) sources. In most of the API MS systems, the ions are generally formed at atmospheric pressure and are carried into a high vacuum chamber where mass analysis of the analyte ions are performed. This process involves the removal of the neutral background gas by large capacity pumps between the ion source and the mass analyzer detector in several stages. Unfortunately, loss of valuable analyte ions between the different pumping stages to a greater extent is unavoidable. An ultimate goal for mass analysis of atmospheric pressure ions would be the removal of the background gas while retaining all of the analyte ions through all of the pumping stages.
To greater extent, the multipole ion guides serve this purpose by capturing the ions and letting the neutral gas be pumped through the rods. This purpose is served better if the ion guide is small and able to go continuously through the different pumping stages and yet minimize the gas flow between the pumping stages. The miniature ion beam guide design, construction, and assembly technique of this invention allows the enrichment of such ions with respect to the background neutral gas. Most mass spectrometers use conical interfaces with small sampling orifice to "skim" ions entrained in the neutral gas expanding into vacuum from atmospheric pressure. The small ion guide design allows the multipole rods to be inserted very close inside the cone across from the sampling orifice allowing more of the ions to be captured without distorting the alternating electric field lines.
If there are four rods per assembly, they are most often used as quadrupole mass analyzers for their ability to filter different mass-to-charge ratio ions. The ideal shape of these rods are hyperbolic; however, in most cases, circular cross sectioned rods can be approximated and are used to generate electric field lines similar to the theoretically ideal hyperbolic field lines between the rods. The Electric field lines are generated by applying AC and DC voltages between the pairs of electrodes which constitute alternating rods in the assembly. If the rod assembly is to be used as an ion guide, only AC voltage is applied to the alternating rods at 180 degrees out of phase from each other. This allows a wide range of mass-to-charge ratio of ions to be stable and transmitted within the ion guide. If a DC voltage is applied between the pair of electrodes in addition to the AC voltage, the multi-rod assemblies are used as a mass filter for a very narrow molecular weight band of ions by adjusting the ratio between the AC and the DC voltages. By keeping the ion guide design small, the electrical capacitance between the rods can be kept to a minimum consuming less power from the resonant driving circuitry.
The overall performance characteristics of an ion guide or a quadrupole mass analyzer judged by its ion transmission efficiency, mass range, sensitivity, and mass resolution is to a high degree determined by the accuracy of the multipole rod assembly. The straightness of the rods, the tolerance build up on all three dimensions of the assembly all play an important role in the accuracy of the results produced by a mass spectrometer. And as the size of the multipole rod assemblies get smaller, it gets harder to maintain the required tolerance levels. In larger rod assemblies conventional machining, welding, brazing and soldering practices can be used to fasten the rods together to keep desired tolerances. In smaller rod assemblies however, the machining becomes prohibitively more difficult and expensive due to lack of material strength, difficulty of handling, and availability of tooling. Voltage connection to the larger rod assemblies are also simpler to make with variety of fastening and brazing methods than the voltage connections to the smaller rod assemblies without distorting or bending them.
To maintain straightness of multipole rods in an assembly can be a challenging task when rod diameters of one mm and rod lengths of beyond 75 mm are being considered. Simple welding or soldering techniques can be implemented if stainless steel rods were to be considered. They are one of the most readily available, inexpensive, and easy to work with materials. Unfortunately, they are easy to bend and very hard to maintain straightness at desired diameter and length combinations. To satisfy straightness, metallic materials such as molybdenum, tungsten or gold coated quartz are commonly used in the art. However, with the desired rod diameters of one mm or less, it becomes almost impossible to fasten any support brackets or connections to the rods. Machining, welding or spot welding, brazing, or soldering of these materials to, for example, stainless steel disks as support structures would be prohibitively difficult and expensive.
Assuming one can obtain desirably straight rods, then one has to assemble them together very accurately. All six rods have to be parallel to each other from end to end. The spacings between the rods have to be equal on a circle, and the end of the rods must meet on a same plane perpendicular to the length of the rods. Once all of these requirements are met, then the complete assembly has to be aligned with the interfacing ion optic lenses and the mass analyzer.
The present invention recognizes the difficulties of having many features in a single yet a small design and be able to overcome the above mentioned design constraints.